In general, a transmission efficiency of a communication system may be greatly increased without increasing bandwidth and power in a multi-input multi-output (MIMO) method using multiple antennas at a respective transmitter and receiver. The MIMO method is classified into an open-loop method and a closed-loop method. A transmitter does not know channel information in the open-loop method, and the transmitter knows the channel information and transmits an information symbol appropriate to a channel by using the channel information in the closed-loop method.
The open-loop MIMO method is classified into space division multiplexing, transmit diversity, and space-time encoding methods according to a diversity gain and a space division multiplexing gain.
The space division multiplexing method is for transmitting symbols different from each other through respective transmit antennas, and a data rate may be increased according to the number of transmit antennas since information symbols of as many as the transmit antennas are transmitted for one symbol period in the space division multiplexing method. However, a bit error rate is problematically increased since power is divided and supplied to the respective transmit antennas.
In addition, the transmit diversity method is for repeatedly transmitting the same or similar information symbols through the respective transmit antennas. Accordingly, a bit error rate is achieved of less than the bit rate in a case of using a single antenna since the same or similar information symbols are transmitted through various paths and channel fading is effectively overcome in the transmit diversity method. However, it is difficult to achieve a data rate as high as the data rate in the space division multiplexing method since one symbol is transmitted for one symbol period by repeatedly transmitting the same symbols from the respective transmit antennas.
To solve the problems of the space division multiplexing method and the transmit diversity method, the space time coding method with a high data rate and stable symbol transmission has been suggested. The transmitter transmits a transmission symbol column encoded into a space-time matrix to a receiver through multiple antennas in order to increase the data rate in the space time coding method. Among the space time coding methods, a full rate full diversity (FDFR) coding method has been suggested and applied to the MIMO system required for a high data rate. The bit error rate is reduced by increasing the data rate according to the number of transmit antennas and achieving the diversity gain and the coding gain in the FEFR coding method (refer to “A construction of a space time code based on number theory”, IEEE Transaction on Information Theory, vol. 48, No. 3. pp 753-760, March 2002; M. O. Damen, A. Tewfix, J. C. Belfiore).
As described, the space time coding method is suitable for a next generation communication system required for a high data rate since a space time code is designed to maintain the same data rate as the space division multiplexing method while obtaining a maximum diversity gain and a coding gain. However, performance for detecting a signal is reduced since the diversity gain is reduced and a signal to noise ratio of a space division multiplexing symbol is steeply reduced when correlativity of channels is increased.
In addition, the closed-loop MIMO method is classified into a full channel regression method and a partial channel regression method according to a quantity of the channel information returned from the receiver.
An adaptive modulation method that varies modulation methods according to respective channel qualities for respective antennas may be applied since the transmitter knows the channel information in the full channel returning method, and accordingly a maximum channel capacity may be obtained. However, there is a problem in that a lot of frequency resources are wasted for returning the channel information.
In the partial channel regression method, information adapted to channels by using a few frequency resources may be transmitted since the receiver extracts and processes characteristics of a channel and returns the extracted and processed channel to the transmitter. A transmit antenna selection method is a representative example of the partial channel regression method, in which a diversity gain and a signal to noise ratio are obtained by selecting transmit antenna groups of a good channel quality and transmitting an information symbol through the selected antennas (refer to R. W. Heath, A. Paulaj; Antenna selection for spatial multiplexing systems based on minimum error rate, ICC 2001, vol. 7, pp 11-14, June 2002).
According to the transmit antenna selection method, a diversity gain may be obtained to be higher than the transmission diversity method by selecting antennas having a high channel gain and low channel correlativity and transmitting the information through the selected antennas. In addition, an increase of a noise power is effectively prevented and the diversity gain is concurrently obtained since the channel correlativity is increased by combining the transmit antenna selection method with the space division multiplexing method.
Accordingly, a method for applying the space time encoding method of the open-loop MIMO method to the transmit antenna selection method of the closed-loop MIMO method is required.
The information disclosed in this Background of the Invention section is only for enhancement of understanding of the background of the invention, and therefore, unless explicitly described to the contrary, it should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art that is already known in this country to a person of ordinary skill in the art.